1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus capable of forming a color image.
2. Related Background Art
Recently, color image forming apparatuses have remarkably been developed and progressed, and there are many kinds of image forming apparatuses such as ink jet type, heat-transfer type, silver salt electrophotographic type and electrophotographic type. Among them, electrophotographic image forming apparatuses have been most popularized due to its high speed ability and high image quality.
FIG. 7 is a sectional view showing an example of a conventional color image forming apparatus of four drum type. Now, a construction and operation of such an apparatus will be described.
A color original 1 rested on an original support is illuminated by an illumination lamp 2, and a color decomposed image is read by a CCD color line sensor 3. The read image is sent, through a color signal process circuit 4 and a cable 25, to an image process circuit 5, where digital image process is performed. Thereafter, full color image signals for one page is temporarily stored in a memory device 26.
The reason is that a plurality of image forming portions (image forming stations) are provided side by side and at least image data corresponding to a distance between the adjacent image forming portions must be stored in order to form plural color images simultaneously.
The image forming portions (drum stations) are prepared for respective colors, i.e., magenta (M), cyan (C), yellow (Y) and black (K). The respective image forming portions include photosensitive drums 29, 30, 31 and 32, primary chargers 41, 42, 43 and 44, developing devices 33, 34, 35 and 36, transfer chargers 37, 38, 39 and 40, and a cleaning devices 62, 63, 64 and 65.
As a recording material (copy sheet) supplied from a cassette 61 is advance, a tip end of the recording material is detected by a tip end detector (ITOP sensor) 67 disposed at an upstream end of a transfer belt 70 (formed by connecting a PET resin film in an endless form, in this example). As a result, in synchronous with a sheet tip end signal, the image signal for color component already stored in the memory device 26 is read at a proper timing by a timing control circuit (not shown), and the read signal is processed in a second digital image signal processing portion 27. Thereafter, a light beam corresponding to a first color (magenta component) modulated by a semi-conductor laser 57 is reflected by a polygon mirror 28 and reflection mirrors 45, 46, 47 to be illuminated onto the photosensitive drum 29 of the first station, thereby forming a first color latent image.
This latent image is developed by the developing device 33 with magenta color toner to form a magenta toner image. The magenta toner image is transferred, by the transfer charger 37, onto the recording material borne on the transfer belt 70 and conveyed to a transfer portion.
Similarly, in the second, third and fourth stations, cyan, yellow and black toner images are formed, respectively, and these color toner images are transferred onto the same single recording material in a superimposed fashion at respective transfer portions. In FIG. 7, the reference numerals 48 to 50 denote reflection mirrors in the second station, 58 denotes a semi-conductor laser, 51 to 53 denote reflection mirrors in the third station, 59 denotes a semi-conductor laser, 54 to 56 denote reflection mirrors in the fourth station, 60 denotes a semi-conductor laser.
The recording material (sheet) to which the four color toner images were transferred is sent, by a convey belt 14, to fixing rollers 15, 16, where color toners are mixed and fixed onto the sheet to provide a full-color copy. In FIG. 7, the reference numeral 66 denotes an interface circuit.
In such an image forming apparatus, as mentioned above, four drum station-for forming the respective color component images, four laser beam generators (semi-conductor lasers) for forming the latent images and four optical systems including the reflection mirrors for illuminating the laser beams onto the respective photosensitive drums with high accuracy are required.
On the other hand, in an accurate full color copying apparatus, it is required that positional accuracy for overlapping the color component images together is 100 to 150 .mu.m or less both in a main scanning direction and a sub scanning direction. However, it is very difficult to mechanically adjust various elements for forming the images with such accuracy. If such adjustment could be achieved, for example, the adjusted positions are apt to be deviated due to positional deviation caused by thermal expansion of various elements generated by change in temperature, and/or positional deviation caused by mechanical positional accuracy after sheet jam treatment and/or exchange of parts for exchanging damaged or worn photosensitive drum or developing device.
In order to permit automatic adjustment of the positional accuracy for overlapping the color component images, there has been proposed the following technique. That is to say, registration patterns (for example marks "#") for magenta, cyan, yellow and black toners are formed on the respective photosensitive drums 29 to 32. As shown in FIG. 8, the patterns are transferred onto the transfer belt 70 so that the color component patterns 72 are formed on the transfer belt 70 in two rows (rear side and front side) at predetermined interval time. And, at the rear side and front side, the light beams are illuminated onto the patterns, and reflected light beams are read by optical sensors 80, 81 (each comprised of CCD). In this way, deviation amounts and inclination amounts of respective image forming stations in the main scanning direction and the sub scanning direction are detected, and the deviation amounts in the sub scanning direction and the main scanning direction are automatically corrected.
The principle for automatic adjustment of registration in the automatic correcting technique is as follows. As mentioned above, since the transfer belt 70 on which the respective color patterns are formed is normally made of PET resin, as shown in FIG. 9B, in a wavelength area of 400 to 700 nm, light permeability is high (90% or more) and, thus, there is no reflection intensity (i.e., cannot be detected hardly). On the other hand, in a near-infrared area over 700 nm, there is reflection intensity. Carbon black as resistance reducing agent may be added to resin to reduce resistance of base resin of the transfer belt. However, also in such a case, in the wavelength area of 400 to 700 nm, there is no reflection intensity.
On the other hand, regarding the toner from which the patterns 72 are formed, as shown in FIG. 9A, for wavelengths between 400 and 700 nm, yellow toner, magenta toner and cyan toner have high light reflection (i.e., these wavelengths have great reflection intensities. Since the black toner is normally uses carbon black as coloring agent, the black toner does almost not have reflection intensity in the wavelength area of 400 to 700 nm.
As mentioned above, the transfer belt 70 can reflect only light of infrared area including near-infrared ray. Accordingly, by arranging infrared ray cut filters (not shown) below light receiving portions of the sensors 80, 81, above mentioned magenta, cyan and yellow patterns can be read. However, since the black pattern have no reflectable wavelength in the entire area, the black pattern cannot be distinguished from the transfer belt.
In order to detect the black patterns, in the yellow station (third station) immediately before the black station, the solid yellow toner is transferred onto the transfer belt. And, black toner registration correcting (registering) patterns are transferred onto an yellow area defined by the solid yellow toner in an overlapping manner, and the black patterns are detected by the sensors 80, 81 as negative images in the yellow background.
FIGS. 10A and 10B are timing charts showing timings for forming the registration patterns in such an image forming apparatus, and, in particular, FIG. 10A corresponds to registration mark forming timings in the sub scanning direction and FIG. 10B corresponds to registration mark forming timings in the main scanning direction.
The automatic correction in the sub scanning direction is effected as follows. As shown in FIG. 10A, in the sub scanning direction, the number of lines is counted by a counter circuit (not shown) in accordance with time elapsed from rise-up of an output signal ITOP (signal detecting a tip end of the copy sheet) from an ITOP sensor 67. Then, for example, in magenta, after tm time is counted, in cyan, after tc time is counted, in yellow, after ty time is counted, and, in black, after tk time is counted, output signals MTOP, CTOP, YTOP and KTOP are generated, respectively. Image signals are sent in accordance with rise-up times of the signals, and the registration color patterns 72 are formed on the transfer belt 70. Then, as mentioned above, the patterns are detected by the sensors 80, 81, and the count values tm to tk are determined on the basis of the detection timings.
The automatic correction in the main scanning direction is effected as follows. Signals MBD, CBD, YBD, KBD shown in FIG. 10B are detection signals of laser beams for forming latent images for magenta, cyan, yellow and black, which signals are reference signals for determining main scanning positions of the images. The positions of the color images are determined by enable signals MEN, CEN, YEN and KEN generated after pixel count values xm, xc, xy and xk from the laser beam detection signals MBD, CBD, YBD, KBD. Accordingly, also in this case, as mentioned above, on the basis of the timing positions detected by the sensors 80, 81, the respective pixel count values xm, xc, xy and xk may be determined properly.
Incidentally, the automatic correction for inclination can be effected by inclining the reflection mirrors 45, 46, 47 by an actuator (not shown) regarding magenta, for example.
However, in the above-mentioned conventional example, for example, the yellow solid image having optical property different from that of the black toner is transferred onto the transfer belt 70 and the black pattern is transferred onto the yellow solid image. Thus, the background yellow toner and the overlaying black toner cannot be removed completely by a cleaner (not shown), with the result that the black toner and the yellow toner remain on the transfer belt 70, thereby contaminating a rear surface of a next copy sheet.